Throughout this specification, various patents, published patent applications and scientific references are cited to describe the state and content of the art. Those disclosures, in their entireties, are hereby incorporated into the present specification by reference.
For several years, it has been known that neural stem cells exist in the adult mammalian brain. This concept is of particular importance since the adult brain was thought to have very limited regenerative capacity. Moreover, the possibility to use adult-derived stem cells for tissue repair may help to overcome the ethical problems associated with embryonic cell research. Although the generation of neurons and glia can be observed in the adult brain, there is thus far only limited knowledge about stimulation of human neural stem cells in vitro and in vivo.
The first suggestions that new neurons were born in the adult mammalian brain came from studies performed in the 1960s (Altman and Das 1965; Altman and Das 1967). It however took another three decades and refined technical procedures to overthrow the dogma that neurogenesis within the mammalian CNS is restricted to embryogenesis and the perinatal period (for review see (Momma, Johansson et al. 2000); (Kuhn and Svendsen 1999)). Treatment of neural disease and injury traditionally focuses on keeping existing neurons alive, but possibilities now arise for exploiting neurogenesis for therapeutic treatments of neurological disorders and diseases.
The source of new neurons is neural stem cells (NSCs), located within the ependymal and/or subventricular zone (SVZ) lining the lateral ventricle (Doetsch, Caille et al. 1999; Johansson, Momma et al. 1999) and in the dentate gyrus of the hippocampus formation (Gage, Kempermann et al. 1998). Recent studies reveal the potential for several additional locations of NSC within the adult CNS (Palmer, Markakis et al. 1999). Asymmetric division of NSC maintain their number while generating a population of rapidly dividing precursor or progenitor cells (Johansson, Momma et al. 1999). The progenitors respond to a range of cues that dictate the extent of their proliferation and their fate, both in terms of the cell type that they differentiate into and the position that they ultimately take up in the brain.
The NSCs of the ventricular system in the adult are likely counterparts of the embryonic ventricular zone stem cells lining the neural tube whose progeny migrate away to form the CNS as differentiated neurons and glia (Jacobson 1991). NSCs persist in the adult lateral ventricle wall (LVW), generating neuronal progenitors which migrate down the rostral migratory stream to the olfactory bulb, where they differentiate into granule cells and periglomerular neurons (Lois and Alvarez-Buylla 1993). Substantial neuronal death occurs in the olfactory bulb generating a need for continuous replacement of lost neurons, a need satisfied by the migrating progenitors derived from the LVW (Biebl, Cooper et al. 2000). Further to this ongoing repopulation of olfactory bulb neurons, there are forceful indications that lost neurons from other brain regions can be replaced by progenitors from the LVW that differentiate into the lost neuron phenotype complete with appropriate neuronal projections and synapses with the correct target cell type (Snyder, Yoon et al. 1997; Magavi, Leavitt et al. 2000).
In vitro cultivation techniques have been established to identify the external signals involved in the regulation of NSC proliferation and differentiation (Johansson, Momma et al. 1999; Johansson, Svensson et al. 1999). The mitogens EGF and basic FGF allow neural progenitors, isolated from the ventricle wall and hippocampus, to be greatly expanded in culture (McKay 1997; Johansson, Svensson et al. 1999). The dividing progenitors remain in an undifferentiated state growing into large balls of cells known as neurospheres. Withdrawal of the mitogens combined with addition of serum induces differentiation of the progenitors into the three cell lineages of the brain: neurons, astrocytes and oligodendrocytes (Doetsch, Caille et al. 1999; Johansson, Momma et al. 1999). Application of specific growth factors can distort the proportions of each cell type in one way or another. For example, CNTF acts to direct the neural progenitors to an astrocytic fate (Johe, Hazel et al. 1996; Rajan and McKay 1998), while the thyroid hormone, triiodothyronine (T3) has been shown to promote oligodendrocyte differentiation (Johe, Hazel et al. 1996). Enhancement of neuronal differentiation of neural progenitors by PDGF has also been documented (Johe, Hazel et al. 1996; Williams, Park et al. 1997).
The ability to expand neural progenitor cells and then manipulate their cell fate has also had enormous implications in transplant therapies for neurological diseases in which specific cell types are lost. The most obvious example is Parkinson's Disease (PD) which is characterized by degeneration of dopaminergic neurons in the substantia nigra. Previous transplantation treatments for PD patients have used fetal tissue taken from the ventral midbrain at a time when substantia nigral dopaminergic neurons are undergoing terminal differentiation (Herman and Abrous 1994). The cells are grafted onto the striatum where they form synaptic contacts with host striatal neurons, their normal synaptic target, restoring dopamine turnover and release to normal levels with significant functional benefits to the patient (Herman and Abrous 1994) (for review see (Bjorklund and Lindvall 2000)). Grafting of fetal tissue is hindered by lack of donor tissue. In vitro expansion and manipulation of NSCs, however, can potentially provide a range of well characterized cells for transplant-based strategies for neurodegenerative diseases, such as dopaminergic cells for PD. To this aim, the identification of factors and pathways that govern the proliferation and differentiation of neural cell types will prove fundamental.
Ultimately the identification of these proliferative and differentiating factors is likely to provide insights into the stimulation of endogenous neurogenesis for the treatment of neurological diseases and disorders. Intraventricular infusion of both EGF and basic FGF have been shown to proliferate the ventricle wall cell population, and in the case of EGF, extensive migration of progenitors into the neighbouring striatal parenchyma (Craig, Tropepe et al. 1996; Kuhn, Winkler et al. 1997). The progenitors differentiated predominantly into a glial lineage while reducing the generation of neurons (Kuhn, Winkler et al. 1997). A recent study found that intraventricular infusion of BDNF in adult rats stimulates an increase in the number of newly generated neurons in the olfactory bulb and rostral migratory stream, and in parenchymal structures, including the striatum, septum, thalamus and hypothalamus (Pencea, Bingaman et al. 2001). These studies demonstrate that the proliferation of progenitors within the SVZ of the LVW can be stimulated and that their lineage can be manipulated to neuronal and glial fates. Currently the number of factors known to affect neurogenesis in vivo is small and their effects are either undesired or limited.
Therefore, there is a long felt need to identify other factors that can selectively stimulate neural stem cell activity through proliferation of neural progenitors and differentiation into the desired neuronal cell type. This activity would be beneficial for both stimulation of in vivo neurogenesis and culture of cells for transplantation therapy. The present invention demonstrates a role for S1P, LPA and their receptors in the proliferation, differentiation, survival and migration of neural stem cells in vitro and in vivo.